This invention relates generally to apparatus used in plastics injection molding and extrusion operations, and more particularly to apparatus having hardfacing coatings for high wear and corrosion resistance.
Injection molding is a process for forming articles from plastics, wherein a melted plastic is forced under pressure into a die cavity to solidify in the shape and size of the cavity. The charge of plastic material to be injected into the die cavity must be heated and pressurized prior to injection, and in one approach the plastic starting material is fed into a hollow cylinder having a feed screw therethrough. As the feed screw turns, the plastic is forced into a heated zone of the cylinder ahead of a check ring on the head of the screw, so that a predetermined volume of heated, pressurized plastic is prepared for subsequent injection into the die cavity by a forward movement of the screw and check ring within the cylinder. A somewhat related process is plastics extrusion, wherein a mass of heated plastic material is continuously forced from a pressurized cylinder through a die by a feed screw, to exit as a desired shape determined by the cross sectional shape of the die.
Because the economics of plastics processing depend upon attainment of long operating lives for the machinery, it is important that the portions of the machinery in contact with the plastic have high resistance to wear and corrosion by the heated plastic material. This combination of good wear resistance and good corrosion resistance is difficult to attain in many situations, and in particular where plastics which produce hydrogen fluoride and/or hydrogen chloride upon heating are used. These two chemicals, the bases for corrosive hydrofluoric and hydrochloric acids, respectively, can result in greatly accelerated corrosion and corrosion wear in the elevated temperature operating environment within the cylinder.
In such plastics processing machinery, should the inside of the cylinder or the outer surfaces of the feed screw wear away under the influences of corrosion and wear from the plastic, a gap develops between the screw and check ring, on the one hand, and the inside diameter of the cylinder, on the other. Plastic material then can leak back from the pressurized zone, with the result that the necessary pressure for injection molding or extrusion will not be attained. The cylinder and/or screw must then be replaced or refurbished, which is expensive and results in an interruption to the economic production process.
Three principal approaches have been taken to provide cylinders and screws having highly wear resistant and corrosion-resistant surfaces, while at the same time retaining the high component strength at elevated temperatures required for the injection molding operation. In one, the inside of a steel cylinder and the outside surface of a feed screw are nitrided, as by exposing the surfaces to be treated to a nitrogen-containing gas such as ammonia, at elevated temperature. This nitriding process results in a relatively thin layer of hardened steel at the protected surface, the layer having improved wear resistance. In a second approach applicable to protecting the feed screw, the outer surface of the screw to be protected is coated with a wear resistant coating using a plasma spray or other convenient coating process.
Another approach, applicable to an improved cylinder, forms a composite cylinder using a centrifugal casting process, wherein ingredients suitable for forming an inner layer within the cylinder are loaded into a premachined cylindrical outer steel housing, the ends are sealed, the housing is placed into a furnace at a temperature sufficiently high to melt at least a portion of the ingredients but not the housing, and the cylindrical housing is then rotated rapidly about its cylindrical axis to distribute the molten ingredients in a continuous layer about the inside of the housing. Upon cooling, the inner layer is metallurgically bonded to the cylindrical outer housing, and the inner layer may then be machined or honed to form a smooth bore of constant diameter to receive the feed screw and check ring therein.
The centrifugal casting process has proved successful for the manufacture of composite cylinders for use in injection molding and extrusion, and a number of alloys have been developed for use in forming the inner lining of the cylinder. Among the materials developed is a heavy-metal alloy of tungsten carbide particles in a nickel-chromium based matrix. The tungsten carbide imparts hardness to the inner layer, and the matrix material binds the tungsten carbide particles, imparts toughness, and allows the fabrication by the centrifugal casting process. Such alloys are disclosed in U.S. Pat. Nos. 3,836,341 and 4,430,389, whose disclosures are herein incorporated by reference. Iron-based alloys, iron-chromium based alloys, iron-nickel based alloys, cobalt-chromium based alloys, cobalt-nickel based alloys, nickel-iron-boron based alloys, and nickel-chromium based alloys are also used in various situations.
None of these alloys has the necessary combination of corrosion resistance, fluidity in the centrifugal casting range, and hardness for wear resistance, that is required to protect injection molding and extrusion components used to process plastics that produce hydrogen fluoride and hydrogen chloride during processing. Accordingly, there has been a need for an improved alloy for use in protecting composite cylinders and hardfaced feed screws. The present invention fulfills this need, and further provides related advantages.